This invention relates to a linking member for connecting a rope and a chain. An important use of a line consisting of a rope and a chain connected through such a link is for the anchoring or mooring of offshore drilling rigs or vessels, particularly semi-submersible drilling rigs operating at moderate or great sea depths.
Though the advantages of this type of mooring line are generally recognized by experts -- the chain part of the line permitting vertical movement of the drilling rig or vessel under the influence of even high waves without over-stressing of the line, the rope part limiting horizontal displacement -- it is still more usual to employ either a chain alone or a rope alone. The reason for this is the lack of a satisfactory linking member suitable for passing across the fairleads or sheaves required for guiding the line during haul-in and pay-out without subjecting the rope to undue bending stresses at its point of connection to the linking member. It has been attempted to overcome this difficulty by stowing the chain on a service vessel from which the chain is connected to the rope outside the drilling rig or main vessel when the rope is paid out from the latter, but this method is time consuming, expensive and dangerous.
A connecting link has been proposed consisting of a linking body having a portion formed as a spherical segment, which linking body is rigidly attached to the rope while the chain is connected with the said body in such a manner as to be capable within certain limits of swinging freely in all directions about its point of attack, which is located at the center of the sphere. In ideal circumstances the effect of this arrangement would be that the chain would exert no moment of rotation on the linking body which would therefore orient itself in the direction of the rope and consequently would not subject the rope to bending stresses at its point of attachment. In practice, however, the rope in orienting the linking body will have to overcome the friction of that body against the fairlead or sheave across which it is being hauled, and besides the chain, engaging a bar or loop fast with the linking body, will not always be directed accurately towards the center of the sphere. Consequently, a certain blend may still be produced at the point of attachment of the rope, and such a bend, which will continually vary in magnitude and direction during use, may cause injury to the rope. Moreover, the possible angle between the rope and the chain is rather limited so that it will often be necessary to provide an external path for supporting the linking member, which tends to make the line guiding structure, such as an outboard bracket carrying fairleads or sheaves, bulky and heavy.